1. Field of the Invention
The invention relates to soundproofing sandwich structures, and more especially to so-called "passive" honeycomb soundproofing panels operating on the basis of quarter-wavelength tuned Helmoltz resonators.
Ever more restrictive standards are forcing ever greater reductions in the sound emission from commercial aircraft, particularly during takeoff and in the vicinity of airports. This sound emission may attain 155 decibels on takeoff, especially against the inside walls of the pods surrounding turbojet engines and their gas flows, and it results in acoustic excitations of an aerodynamic origin stemming essentially from the engines and aeroacoustic excitations which generate considerable vibro-acoustic phenomena both with regard to the structures and the cabin. The sound emission occurs primarily in a fairly wide frequency range stretching from 200 to 8000 Hz, and more particularly between 600 and 5000 Hz where the annoyance is greatest. However, the current trend is toward the general use of so-called high bypass ratio fan-jet engines which allow substantial reductions in the consumption of fuel to be achieved. These turbojet engines have a lower speed of rotation, and consequently their sound emission is extended more toward the low frequencies.
In this context, designers and manufacturers of specific aeronautical components are searching for soundproofing treatments which make is possible to reduce the acoustic levels without penalizing other aspects such as mechanical strength, weight and bulk. In particular, a reduction in bulk can be achieved by soundproofing panels of lesser thickness, thus making it possible to thin down the structures of the aircraft, particularly the pods surrounding the engines, and consequently to reduce the drag of the aircraft and its fuel consumption.
2 Summary of the Prior Art
The known soundproofing panels with the highest performance in terms of acoustic effectiveness, weight and bulk are formed from a sandwich structure comprising a honeycomb core whose cells are arranged as Helmoltz resonators. The behavior of the panel can be represented by its reduced normalized acoustic impedance with respect to the impedance of air .rho.c: EQU Z=R+j.X
The real term R expresses its resistive component and should be close to one for better absorption of sound at middle frequencies. The complex term X expresses its reactive component and should be a minimum so as to avoid absorption-free reflection of the sound at the bottom of the soundproofing panel.
It is known practice to make dual Helmoltz resonator soundproofing panels comprising, in succession, a solid skin, a honeycombed core whose cells are divided in the direction of its thickness into two cavities by a thin partition made of resin and perforated right the way through it, and a porous skin which communicates with the sound source. The partition is positioned so as to tune the resonance according to two chosen frequencies, the air molecules moving alternately from one cavity to the other via the perforations through the partition, and energy dissipation taking place through the viscous laminar flow of the air at the perforations. The perforations are usually formed by laser beam, since they should preferably be of small size and high density. However, it is known that these perforated partitions exhibit a considerable nonlinearity with fast displacement speeds resulting from high acoustic levels. Therefore, a panel which is optimized at low sound levels, for example at 120 decibels, will experience a degraded performance at high sound levels, for example at 155 decibels, and vice versa. Moreover, the optimization of such soundproofing panels leads to a ratio between the total area of the perforations and the surface area of the partition, referred to as the "perforation ratio" or "porosity", of the order of 2% to 5%. Hence, the speed of the air molecules increases greatly on passing through the partition, thereby increasing the nonlinear behavior of the partition.
It is also known practice to make soundproofing panels of the same type in which the partition is formed by a fabric of metal wires sandwiched between two honeycomb layers. This partition offers better linearity combined with greater porosity, i.e. of the order of 20%, such porosity engendering a lesser increase in the speed of the air molecules passing through the partition and therefore improving the linearity. However, such a panel exhibits a proneness to delamination between the two honeycomb layers and the fabric.
It is understood that the dividing of the honeycomb into two layers is effected merely for purposes of feasibility and cost.
It is also known practice to make soundproofing panels consisting of an absorbent filler sandwiched between two skins, one at least of which is porous. The filler may be provided by fiber, by an open-pored foam, or more recently by microbeads, hollow microbeads with porous walls seeming to give the best results. However, such panels should have a thickness approaching substantially a quarter of the wavelength of the sound at the lowest frequency to be absorbed, and in aeronautics this would lead to prohibitive bulk. Moreover, these panels would be very heavy since their effectiveness depends on the density of the absorbent filler.